The present invention relates to an improvement of a control circuit for use in electrically powered tools, such as screw driver, a drill or the like.
A conventional electrically powered tool is shown in FIG. 1. An arrangement and operation of the tool is described in detail in U.S. Pat. No. 5,019,023 to Kurosawa, so that a detailed description thereof is omitted herein. Briefly, the tool 31 has a clutch 33 axially disposed between a motor shaft 35 and an output shaft 34. The clutch 33 is coupled at one side to the motor shaft 35 and at opposite side to the output shaft 34. When a load greater than a predetermined level determined by a tightening spring 32 is imparted on the output shaft 34 during drilling or screwing, the output shaft 34 is decoupled from the clutch 33 with an axial movement of the clutch 33. An interlocking switch 7 is provided which performs ON and OFF actions in interlocking relation with the actuation of the clutch 33. When the clutch 33 is decoupled from the output shaft 34, the switch 7 is rendered ON, and in response thereto rotations of the motor are stopped.
FIG. 2 is a circuit diagram showing a control circuit for stopping the rotations of the motor. In the circuit shown therein, a d.c. power supply 1, a main switch 2, a single-pole relay switch 10 and a motor 4 are connected in series. A series connection of a relay coil 5 and a thyristor 6 is connected in parallel across the relay switch 10 and the motor 4. A series connection of the switch 7 and resistors 8 and 9 is further connected in parallel across the relay switch 10 and the motor 4.
When the switch 7 is rendered ON, the relay coil 5 is energized, thereby causing a wiper contact of the relay switch 10 to trip over to OFF position. However, the wiper contact of the relay switch 10 is liable to undergo chattering or fluttering actions at the OFF position, during which a large braking current flows in the relay switch 10, resulting in melting of the contact of the relay switch 10.